This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cell adhesion in the presence of hemodynamic flow plays a fundamental role in many of the physiological processes occurring in the circulatory system. These adhesive interactions are influenced by processes taking place at drastically different length scales: (1) Fluid forces which tend to deform the cells are on the length scale of the cell diameter (~10 ?m) whereas (2) the range of receptor-ligand bonds mediating cell adhesion is ~ 100 nm. The coupling of forces acting at these disparate length scales as well as their effects on cell deformation makes the simulation of cell adhesion computationally intensive. We simulate the homotypic aggregation of polymorphonuclear leukocytes in a linear shear field using the immersed boundary method with a stochastic description of receptor-ligand interactions. The cells have been modeled as spherical Neo-Hookean membranes enclosing a Newtonian fluid. This study will shed light on the effects of fluid shear and cell deformability on the efficiency of leukocyte aggregation.